The role of biological and chemical inducers in activating bean defense responses against Rhizoctonia solani

Rhizoctonia root rot is an important bean disease that causes significant yield losses. Increasing the levels of plant resistance using biological and chemical inducers is a sustainable strategy for plant disease management. In this study, the efficiency of thiamine, riboflavin, pyridoxine, folic acid, menadione, quercetin, naringenin, humic acid, fulvic acid, boric acid, zinc sulfate, manganese sulfate, Piriformospora indica and binucleate Rhizoctonia were evaluated to improve bean resistance to root rot pathogen Rhizoctonia solani AG-4 HGII. Under laboratory conditions, effect of various biological agents and chemical compounds on mycelial growth of the pathogen was evaluated. Results showed that applying boric acid, zinc sulfate and manganese sulfate at 1000 and 2000 mg L-1 concentrations and menadione at 2000, 5000, 10000 and 20000 mu M completely inhibited the pathogen growth and 10000 and 20000 mu M concentrations of thiamine reduced linear growth of R. solani. Under greenhouse conditions, application of the inducers significantly reduced the disease progress on treated plants compared to the non-treated plants. The highest level of disease reduction was observed using zinc sulfate and thiamine applied at 3 days before the pathogen inoculation, and also by using P. indica 14 days before inoculation with the pathogen. Thiamine, zinc sulfate and P. indica significantly reduced the disease index and improved plant growth parameters compared to the controls. Induction of resistance against R. solani by individual and combined treatments using thiamine, zinc sulfate and P. indica was associated with increased activity of various antioxidants and increased levels of membrane stability index (MSI), relative water content (RWC), lignin accumulation and total soluble phenolics in bean seedlings. Our findings revealed a practical possibility of inducing resistance and protection of bean plants against Rhizoctonia root rot with the help of biological and chemical resistance inducers.